Two types of MRI apparatus are currently known in the art, differing both in cavity size and in the intensity of the static magnetic field applied for nuclear spin orientation. These two types of apparatus are known as “total body” or “dedicated” scanners.
“Total body” scanners are designed for examination of entire bodies and have a large size, required for receiving the whole body under examination, with magnetic fields of medium to high intensity, and magnet structures requiring complex, large and expensive cooling systems.
Dedicated scanners are smaller and of easier installation, due to their low space requirements and weight, and their imaging cavities are of a size adapted to receive and examine only certain parts or a body and only certain anatomic regions thereof: typically, in these scanners static magnetic fields are generated by permanent magnets.
Nevertheless, these dedicated scanners are still of relatively large size, considering that even a magnetic field of low-to-medium intensity requires a relatively large amount of magnetized material to reach said intensity levels, and the magnet structure requires a bearing structure and elements such as yokes and/or ferromagnetic, as needed. Therefore, their reduced cavity size, allowing limited use with certain parts of a body only, does not correspond to an equivalently reduced size of the magnet structure and the MRI apparatus.
Furthermore, such low-to-medium static magnetic field causes the apparatus to be more sensitive to external noise fields, because the signals emitted from the body under examination are of relatively low intensity.
In addition to the above mentioned size-related problem, prior art MRI apparatus are characterized by magnet structures symmetrical with respect to the center of their own magnetic cavity; such symmetry inevitably involves a symmetric configuration of the magnetic field generated within said structure, so that the geometric center of the geometric cavity coincides with the center of the imaging volume, which is, defined as the point in which the gradient of the magnetic field generated within said magnet structure has a zero value.
This configuration is particularly disadvantageous in many clinical applications, in which the region of interest does not coincide with the geometric center of the body part under examination that contains the region of interest: examples of these applications may be examinations of radiologically important body parts, such as the rachis or the wrist.
In view of these considerations, the need arises for a solution that allows the region of interest of the human body to be examined to fall within the imaging volume, without increasing the size of the magnet structure.
The invention fulfills the above objects by providing an MRI apparatus as described hereinbefore which, using simple and inexpensive arrangements, can offset the geometric center of the cavity delimited by the magnet structure to the geometric center of the imaging volume.
As described in document EP0187691, the term imaging volume designates a region in the space contained in the cavity, in which region the static magnetic field generated by the generating means has a given homogeneity within given tolerances.
Therefore, this volume portion contained in the cavity is delimited by a non material boundary surface, which is determined by the features of the magnetic field, that is generally provided as a volume of spheroidal shape, or any shape allowing determination of its geometric center.
Both the magnet structure of the present invention and prior art magnet structures have such a configuration that they delimit a patient receiving cavity with regular geometric shapes, in which a geometric center is easily determinable.
As a rule, as described in document EP0187691, this geometric shape is a cylindrical shape with base surfaces parallel to the static magnetic field generating means, consisting of magnetic pole pieces, and whose geometric center is a point on the axis that passes through the center of the base surfaces and is perpendicular thereto and equidistant from the magnetic pole pieces.
As a result, the offset of the imaging volume center relative to the geometric center of the cavity delimited by the magnet structure occurs in the direction of this axis, whereby the points that correspond to the center of the imaging volume and the geometric center of the cavity are aligned but not coincident.